The present invention relates to windshield wipers and more particularly to an improved universal refill unit for windshield wipers.
Windshield wipers are generally supported at their central portion by a spring biased arm attached centrally to the superstructure which transmits the applied pressure through a plurality of longitudinally spaced opposed claws to a flexible backing member. The flexible backing member supports an elastomeric squeegee or wiping element and distributes the applied pressure over the entire length thereof. In conventional blades the pressure distribution pattern varies considerably over the length of the blade. Generally pressure is greatest in the area which is adjacent to the claws and decreases to a minimum value in zones which are located between successive pairs of claws. Lack of an appropriate pattern of pressure distribution can result in occurrence of streaks and unwiped areas on the windshield. Among the causes of these problems are failure of the blade to contact the windshield and also windlift from the windstream when pressure at certain points on the wiping element is inadequate. Excessive pressure is also undesirable in that it produces excessive wiper blade layover and consequent smearing. The ideal is to achieve uniform pressure distribution at an optimum pressure which will cause the edge of the lip to engage the windshield with sufficient force to avoid windlift but to avoid excessive layover and friction. Wiper blade assemblies and refill units have been proposed designed to compensate for variations in pressure distribution in a number of ways.
U.S. Pat. No. 3,473,186 to Mainka discloses a refill unit in which the rubber shoulder portion of a blade is decreased in width where the claws engage it to relieve the pressure at those points. It would tend to equalize the distribution of pressure; however such a design would only be applicable to a specific superstructure for which it is designed. The U.S. Pat. No. to Appel, 3,192,551, discloses a single leaf spring type superstructure varying in width from center to end to provide uniform pressure loading. This design relates to a complete blade; it is not intended to serve as a refill unit for conventional superstructures. The U.S. Pat. No. to Journee 3,969,784 discloses a superstructure with specially designed claws for equalizing the pressure distribution.
The refill unit comprising the backing element and the elastomeric squeegee element is an expendable unit which wears out with use. Generally it is removable from the original superstructure and replaceable. The drawbacks in the above-mentioned designs are their lack of universality. In each case either a complete new blade or a refill unit specifically designed for each particular superstructure would be required. For the sake of economy and availability it is desirable for a refill unit to be adaptable to various types of superstructures currently in use.
Certain types of wiper blade assemblies currently in use incorporate a full length superstructure in which the outboard claws engage the ends of the elongate refill unit adjacent the ends of the backing element. In that type of blade substantially greater pressure is transmitted to the ends of the blade than to the central portion which could result in streaking at the central portion and smearing at the ends. To compensate for the undesirable pressure distribution resulting from such a lever system the backing strip is formed with a longitudinal convex curvature on its side facing the squeegee element. Pressure is transferred from the ends to the central portion of the blade to thereby provide a pressure distribution pattern which avoids missed areas, streaking and smearing.
In certain other types of blade assemblies the outboard claws of the superstructure engage the backing strip at points spaced a substantial distance from the ends thereof. Thus, insufficient pressure may be transmitted to the ends of the squeegee element resulting in missed wiped areas at the ends. To compensate for this type of pressure distribution the backing elements are formed with a longitudinal concave curvature on the side facing the squeegee element hereinafter referred to as a negative bow, thereby increasing the pressure at the ends of the refill unit and relieving it somewhat in the central portion.
A compromise refill unit has been developed which is adaptable to both full length and short superstructures. This involves the incorporation of a backing strip convex on its surface facing the squeegee element but to a much lesser extent than the curvature of the backing strip designed for a full length superstructure. This produces a usable refill unit for both full length and short superstructures. Although adequate this compromise refill unit produces a less desirable pressure distribution pattern than the above mentioned units specifically designed for either full length or short superstructures.